The present invention relates to a magnetic recording medium having a lubricant layer on a magnetic layer or on an inorganic protective layer formed over a magnetic layer. More particularly, the present invention relates to a metal thin film type magnetic recording medium which is excellent in running properties, durability and shelf stability.
Magnetic recording mediums, e.g. magnetic tapes, floppy disks, etc., are commonly coated at the magnetic layer surface thereof with a lubricant to improve lubrication between the magnetic recording medium and the magnetic head and to enhance the running durability of the magnetic recording medium.
With the achievement of high recording density, the magnetic layer surfaces of magnetic recording mediums have become smoother, and the coefficient of friction has become extremely large. Thus, durability concerning the sliding contact with equipment has become a serious problem. Further, magnetic recording mediums are used under various environmental conditions, and information recorded thereon are usually stored for a long period of time ranging from several years to several tens of years. Accordingly, magnetic recording mediums are demanded to have running durability and shelf stability under a wide variety of environmental conditions. Under these circumstances, conventional lubricants have become unable to produce satisfactory effect.
Further, small-sized video tape recorders such as cam coders and 8-mm VTR are often used outdoor. Therefore, magnetic recording mediums are demanded to endure use in a wide range of environmental conditions.
To improve magnetic properties required for high-density magnetic recording, e.g., high-definition recording, digital recording, etc., metal thin film type magnetic recording mediums that use a ferromagnetic metal thin film as a magnetic layer and that are useful for high-density magnetic recording have been expected to replace the conventional magnetic recording mediums. However, in a metal thin film type magnetic recording medium, the magnetic layer is protected by only an extremely thin metallic oxide layer. Therefore, it has been demanded to provide a lubricant layer material which is capable of not only ensuring running properties and durability but also markedly improving shelf stability of metal thin film type magnetic recording mediums. Further, metal thin film type magnetic recording mediums are being improved so as enable achievement of higher-density recording by smoothing the magnetic layer surface and also changing the composition of the magnetic layer from CoNi--O to Co--O or Co--Fe that contains Co--O, etc. so as to attain high coercive force (Bm). With the conventional metal thin film type magnetic recording mediums that have a high cobalt content, however, it has heretofore been difficult obtain stable running properties, durability and corrosion resistance.
Studies have also been conducted of a method for improving durability by forming on the magnetic layer a protective layer of, for example, silica, alumina, titania, graphite, amorphous carbon, cobalt oxide, titanium nitride, chromium carbide, etc. With this method, however, the thickness of the protective layer must be increased in order to obtain satisfactory protective action, resulting in spacing loss. Accordingly, the output reduces, and the achievement of high recording density is hindered, unfavorably.
Under the above-described circumstances, hydrocarbon- or fluorine-containing lubricants which have various structures, and which are excellent in lubricating performance have been examined as being lubricants for metal thin film type magnetic recording mediums, and it is known that an organic fluorine compound having a hydrophilic functional group in a molecule thereof is capable of improving running durability to a considerable extent.
However, even if a lubricant having excellent characteristics is used, a smooth metal thin film type magnetic recording medium having improved electromagnetic transducing characteristics suffers from the problem that the lubricant attached to the surface of the magnetic layer is gradually lost because of repeated sliding movement, resulting in deterioration of the characteristics.
One approach that has heretofore been made to improve the characteristics is to introduce various polar groups into the lubricant. For example, Japanese Patent Application Laid-Open (KOKAI) No. 59-119537 (U.S. Pat. No. No. 4,537,832) and Japanese Patent Application Post-Examination Publication No. 04-50644 disclose a technique in which a carboxyl group, an ester group, and a phosphoric ester are introduced. The disclosed technique involves, however, the problem that repeated running durability at low temperature is inferior. There have also been made many studies to improve the characteristics by using two or more different kinds of lubricant in combination.
Japanese Patent Application Laid-Open (KOKAI) No. 62-141625 discloses the use of a carboxylic acid and a fluorine-containing ester in combination. With this method, the repeated running durability at low temperature can be markedly improved in comparison to a case where each lubricant is used alone. However, the magnetic recording medium using a carboxylic acid and a fluorine-containing ester in combination has the problem that it is inferior in corrosion resistance, particularly shelf stability under high-temperature and high-humidity conditions.
Japanese Patent Application Laid-Open (KOKAI) Nos. 62-236120 (U.S. Pat. No. 4,735,848) and 62-103824 disclose the use of a phosphoric triester or a phosphoric diester and a fluorine-containing ester lubricant. However, since triesters and diesters exhibit weak adsorption to a magnetic film, the wear resistance cannot be improved. Therefore, satisfactory characteristics cannot necessarily be obtained.
Japanese Patent Application Laid-Open (KOKAI) No. 04-205712 proposes a method in which a lubricant layer is formed from two different kinds of lubricant, and in which the amounts of lubricant at the magnetic layer side and at the back coat layer side are specified. With this method, however, no satisfactory lubricating properties can be obtained.
Japanese Patent Application Post-Examination Publication No. 57-29767 and Japanese Patent Application Laid-Open (KOKAI) Nos. 58-188326, 60-63711, 60-63712, 62-209718 and 01-211215 disclose a method in which a lubricant is applied to a back coat layer provided on the side of a magnetic recording medium which is reverse to the side thereof on which a magnetic layer is formed, thereby enabling the lubricant to be appropriately supplied from the back coat layer to the magnetic layer surface where the lubricant is gradually lost by sliding movement. Such a coating method enables an improvement in repeated running durability. However, when a lubricant is coated on only the back coat layer, durability is difficult to ensure; when a lubricant is coated on both the back coat layer and the magnetic layer, a surplus of lubricant is inevitably present on the magnetic layer. Consequently, the coefficient of static friction rises, giving rise to problems such as undesired sticking of the magnetic recording medium.
In terms of corrosion resistance, which is a problem in practical use of magnetic recording mediums that use a ferromagnetic metal thin film as a magnetic layer, the required corrosion resistance cannot sufficiently be ensured by only a fluorine-containing lubricant having a polar group in a molecule thereof. Accordingly, it has been proposed to use such a lubricant in combination with a rust preventive. However, it has heretofore been difficult to ensure the required corrosion resistance by the combined use of a lubricant and a rust preventive that are selected from those which are generally known.
An object of the present invention is to provide a magnetic recording medium which has the coefficient of friction stabilized at a low level, and which is excellent in repeated running durability and corrosion resistance and capable of high-density magnetic recording in particular.